Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery-of-choice for general purpose uses such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware whether industrial or military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multicell structure. Each cell contains a plurality of vertical positive and negative plates formed of lead-based alloy grids containing layers of electrochemically active pastes. This battery has been widely used in the automotive industry for many years, and there is substantial experience and tooling in place for manufacturing this battery and its components. The battery is based on readily available materials, is inexpensive to manufacture and is widely accepted by consumers.
The open circuit potential developed between each positive and negative plate is about 2 volts. Since the plates are connected in parallel, the combined potential for each cell will also be about 2 volts regardless of the number of plates utilized in the cell. One or more cells are then connected in series to provide a battery of desired voltage. The bus bars and top straps used for intercell connection add to the weight and the cost of the battery and often are subject to atmospheric or electrochemical corrosion at or near the terminals.
Another problem with lead-acid batteries is their limited lifetime due to shedding of the active materials from the vertically oriented positive and negative plates. After a period of operation, sufficient flakes accumulate to short circuit the grids resulting in a dead battery cell and shortened battery life.
Lead-acid batteries are inherently heavy due to use of the heavy metal lead in constructing the plates. Modern attempts to produce lightweight lead-acid batteries, especially in the aircraft, electric car and vehicle fields, have placed their emphasis on producing thinner plates from lighter weight materials used in place of and in combination with lead. The thinner plates allow the use of more plates for a given volume, thus increasing the power density. Higher voltages are provided in a bipolar battery including bipolar plates capable of through-plate conduction to serially connect electrodes or cells. Horizontal orientation of the grids prevents the accumulation of flake lead compounds at the battery bottom. Downward movement of electrolyte can be blocked by use of porous glass mats to contain the electrolyte. Also stratification of electrolyte is prevented since the electrolyte is confined and contained within the acid resistant mats by capillary action.
The bipolar plates must be impervious to electrolyte and be electrically conductive to provide a serial connection between electrodes. The bipolar plates also provide a continuous surface to prevent sluffing off of active materials from the grids. Most prior bipolar plates have utilized metallic substrates such as lead or lead alloys. The use of lead alloys, such as antimony, gives strength to the substrate but causes increased corrosion and gassing. Alternate approaches have included plates formed by dispersing conductive particles or filaments such as carbon, graphite or metal in a resin binder.
Some more recent examples of batteries containing bipolar plates are U.S. Pat. No. 4,275,130 in which the biplate construction comprises a thin composite of randomly oriented conductive graphite, carbon or metal fibers imbedded in a resin matrix with strips of lead plated on opposite surfaces thereof. Ser. No. 279,841, filed July 2, 1981, discloses a biplate formed of a thin sheet of titanium covered with a conductive, protective layer of epoxy resin containing graphite powder.
It has been attempted to increase the conductivity and strength of bipolar plates by adding a conductive filler such as graphite. Graphite has been used successfully as a conductive filler in other electrochemical cells, such as in the manganese dioxide, positive active paste of the common carbonzinc cell, and it has been mixed with sulfur in sodium-sulfur cells. However, even though graphite is usually a fairly inert material, it is oxidized in the agressive electrochemical environment of the positive plates in the lead-acid cell to acetic acid. The acetate ions combine with the lead ion to form lead acetate, a weak salt readily soluble in the sulfuric acid electrolyte. This reaction depletes the active material from the paste and ties up the lead as a salt which does not contribute to production or storage of electricity. Acetic acid also attacks the lead grids of the positive electrodes during charge ultimately causing them to disintegrate. Highly conductive metals such as copper or silver are not capable of withstanding the high potential and strong acid environment present at the positive plate of a lead-acid battery. A few electrochemically inert metals such as platinum are reasonably stable. But the scarcity and high cost of such metals prevent their use in high volume commercial applications such as the lead-acid battery. Platinum would be a poor choice even if it could be afforded, because of its low gassing over-potentials.
A low cost, lightweight stable bipolar plate is disclosed in copending application Ser. No. 346,414, filed Feb. 18, 1982, for BIPOLAR BATTERY PLATE. The plate is produced by placing lead pellets into apertures formed in a thermoplastics sheet and rolling or pressing the sheet with a heated platen to compress the pellets and seal them into the sheet. This method involves several mechanical operations and requires that every aperture be filled with a pellet before heating and pressing in order to form a fluid impervious plate. An improved, stable bipolar plate containing a dispersion of conductive tin oxide in resin is disclosed in application Ser. No. 550,875, filed concurrently herewith.
Another limitation on the service life of lead-acid batteries is that during discharge, the lead dioxide (a fairly good conductor) in the positive plate is converted to lead sulfate, an insulator. The lead sulfate can form a thin, impervious layer encapsulating the lead dioxide particles which limits the utilization to less than 50% of capacity, typically around 30%. Furthermore, the lead sulfate can grow into large, hard, angular crystals, disrupting the layer of paste on the grid resulting in flaking and shedding of active material from the grid. Power consumption during charge is also increased due to the presence of lead sulfate insulator. Even when very thin pastes are utilized, the coating of insulating lead sulfate interferes with power output.
An apparent solution to this problem would be the addition of a conductive filler to the paste. The filler must be thermodynamically stable to the electrochemical environment of the cell, both with respect to oxidation and reduction at the potential experienced during charge and discharge of the cell, and to attack by the acid. As previously discussed, a conductive filler such as graphite can not be utilized since graphite is not thermodynamically stable in the oxidizing environment of the positive electrode.
An improved lead-acid paste containing conductive tin oxide is disclosed in copending application Ser. No. 488,199, filed Apr. 25, 1983. The positive active material retains conductivity during both charge and discharge cycles, and the power output of the battery is more uniform since it is less dependent on the state-of-charge and more nearly approaches theoretical efficiency.
The paste does not have good structural strength by itself. A grid or other reinforcement such as a sheet of glass scrim is utilized to provide structural integrity for the layer of cured paste. The interface between the paste and a through-conductive plate, especially a composite plate of resin and filler, introduces resistance into the cell and battery. Furthermore, the plate and the paste require separate manufacturing steps and assembly to form a bipolar plate assembly.